Nanopartide aspect ratio

The aspect ratio of a solid is defined as its length divided by its width therefore, spheres have an aspect ratio of 1. We define, somewhat arbitrarily, a nanorod to be an object with an aspect ratio between 1 and 20, with the short dimension on the 10-100 nm scale, and a nanowire to be an object with an aspect ratio greater than 20 (with the short dimension on the 10-100 nm scale) [20]. The extinction spectra (the combination of visible absorption and scattering) of silver and gold nanopartides are tunable throughout the visible, depending on the aspect ratio [5-10 Figure 9.1]. 

In addition to monosized nanopartide, the morphology of binary nanopartides filled in microphase separating AB diblock copolymers has also been studied. For chemically identical spherical partides, large particles are concentrated in the preferred, compatible phase and small partides spread out in the interfacial regions and in the incompatible phase of the diblock copolymer [17]. Theoretical efforts have also been made toward the morphology of copolymer filled with anisotropic particles (e.g., rod-like partides, plate-like partides, or the mixtures of particles of different shapes). It is found that the distribution of partides within the copolymers depends not only on the relative interaction energies between nanopartides and different blocks but also on the aspect ratio of the rod-like nanopartides. 

Mechanical properties of polymer nanocomposites can be predicted by using analytical models and numerical simulations at a wide range of time- and length scales, for example, from molecular scale (e.g., MD) to microscale (e.g., Halpin-Tsai), to macroscale (e.g., FEM), and their combinations. MD simulations can study the local load transfers, interface properties, or failure modes at the nanoscale. Micromechanical models and continuum models may provide a simple and rapid way to predict the global mechanical properties of nanocomposites and correlate them with the key factors (e.g., particle volume fraction, particle geometry and orientation, and property ratio between particle and matrix). Recently, some of these models have been applied to polymer nanocomposites to predict their thermal-mechanical properties. Young s modulus, and reinforcement efficiency and to examine the effects of the nature of individual nanopartides (e.g., aspect ratio, shape, orientation, clustering, and the modulus ratio of nanopartide to polymer matrix). 

Effective medium theory (EMT) is commonly used to describe the microstructure-property relationships in heterogeneous materials and predict the effective physical properties. It has recently been revised to predict the thermal conduction of nanocomposites. For nanocomposites with nanopartides on the order of or smaller than the phonon mean free path, the interface density of nanopartides is a primary factor in determining the thermal conductivity. In graphite nanosheet polymer composites, the interfacial thermal resistance still plays a role in the overall thermal transport. However, the thermal conductivity depends strongly on the aspect ratio and on the orientation of graphite nanosheets. 

There is a general agreement that nanocomposites imply nanoscale fillers with sizes less than 100 nm in at least one dimension. The fillers can be classified into three groups depending on their shape. One-dimensional nanofillers are nanorods, fibers, or tubes with varying aspect ratios. Rodlike nanopartides can introduce anisotropic properties to composite materials, for example, silver or gold nanorods are used for preparation of dichroic nanocomposites as described later (Figure 1). 

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